Candidate: Jonathan Mansbach, MD is an Instructor in Pediatrics at Harvard Medical School at Children's Hospital Boston (CHB). He supervises an inpatient pediatric service and works with his mentor, Carlos A. Camargo, MD, DrPH, conducting bronchiolitis research. As Co-PIs, they recently completed two Emergency Department (ED)-based bronchiolitis studies: 1) a 37-site cohort study with data collection on 2,847 children, and 2) a 14-site virology study with data collection on 277 children. He also has published health services research on bronchiolitis. Finally, he is a Co-Investigator on U01 AI67693 (Camargo, PI), a 15-site study of 2,250 inpatients age <2 years with bronchiolitis. This K23 application will build directly on this U01 project. Training: In order to meet his career goals of: 1) becoming a national leader of bronchiolitis research;2) understanding the association between vitamin D deficiency and severity of bronchiolits;and 3) becoming an independent patient-oriented investigator, Dr Mansbach requires more training in epidemiology, biostatistics, vitamin D /nutrition, and infectious disease. Therefore, he proposes to take masters-level courses at the Harvard School of Public Health in all of these areas. He also will take classes in leadership development and the ethical conduct of research. Science: Our research has shown that vitamin D deficiency is associated with increased respiratory infections in the first 3 months of life and increased risk of childhood wheezing. The scientific portion of this K23 application focuses on the association between vitamin D deficiency in infancy and the severity of bronchiolitis. Dr Mansbach proposes to conduct a study of children with bronchiolitis presenting to CHB to examine if vitamin D deficiency is associated with: 1) the level of care a child requires - sent home from the ED, ward admission, or intensive care unit admission;2) the viral load of respiratory syncytial virus;and 3) the duration of cough and hospital length of stay. Using 75 hospitalized patients from the U01, and 150 newly-enrolled ED patients for this K23 project, we will have 80% power to detect reasonable odds ratios, effect sizes, and hazard ratios for each of the hypotheses. Summary: A multidisciplinary team of experienced mentors and consultants have committed to help Dr Mansbach fulfill the personal and scientific goals of this K23 application. The results from the scientific portion of this proposal may provide data that will change the care of children and directly lead to R01 applications.